1. Field of the Invention
This invention relates generally to the field of compression relief engine brake for internal combustion engines. More particularly, it relates to a pressure relief system which automatically disables, one or more operating cylinders of, the compression relief engine brake whenever the forces in the hydraulic circuit of the engine brake exceed a predetermined level.
2. Prior Art
Engine brakes of the compression relief type are well known in the art. Such engine brakes are designed to convert, temporarily, an internal combustion engine of the spark ignition or compression ignition type into an air compressor so as to develop a retarding horsepower which may be a substantial portion of the operating horsepower normally developed by the engine.
As a general rule, so long as the retarding horsepower developed during braking operations does not exceed in absolute value the operating horsepower for which the engine was designed, the stresses on the crankshaft, bearings and drive train, though opposite in direction will not exceed the allowable stresses for these parts and the addition of the compression relief engine brake will not adversely affect the operating life of the drive train components of the engine and vehicle. At the same time, the engine brake will supplement the braking capacity of the primary vehicle wheel braking system and extend, substantially, the life of the primary braking system. The basic design for an engine braking system of the type here involved is disclosed in the Cummins U.S. Pat. No. 3,220,392.
The compression relief engine brake of the type disclosed in U.S. Pat. No. 3,220,392 employs a hydraulic system wherein the motion of a master piston controls the motion of a slave piston which opens the exhaust valve of the internal combustion engine near the end of the compression stroke whereby the work done in compressing the intake air is not recovered during the expansion or "power" stroke but, instead, is dissipated through the exhaust and radiator systems. The master piston is customarily driven by a pushrod controlled by the engine camshaft. It will be apparent that the force required to open the exhaust valve will be transmitted back through the hydraulic system to the pushrod and camshaft. In order to minimize modification of the engine, it is common to utilize an existing pushrod which moves at the appropriate time to operate the engine brake hydraulic system. In some cases, an exhaust valve pushrod is selected while, in other cases, it is convenient to use the fuel injector pushrod.
However, by assigning a second function to an existing pushrod, the possibility exists that an increased load which may exceed the design capacity of the pushrod or camshaft may be experienced. In order to avoid damage to the engine pushrod or camshaft, it is desirable to provide an automatic means to unload the engine brake whenever an excessive loading condition becomes imminent. But it is also important automatically to reactivate the engine brake as soon as the temporary excess loading condition has terminated so as not to interfere with the effectiveness of the engine brake.
It has been known to provide means to unload the brake hydraulic system when excess motion of the exhaust valve occurs, see Laas U.S. Pat. No. 3,405,699. Similarly, quick opening relief or check valves of various designs have been disclosed in a number of patents including Parker U.S. Pat. No. 2,431,769, Frain U.S. Pat. No. 2,793,656, Glass et al U.S. Pat. No. 2,817,356, Kelly U.S. Pat. No. 2,874,718, Price U.S. Pat. No. 3,194,260, Trick U.S. Pat. No. 3,199,532, Chapman et al U.S. Pat. No. 3,589,386 and Hammer et al U.S. Pat. No. 3,651,827.